Add tests for UnusedPropertyRule

Summary

Adds 14 unit tests for UnusedPropertyRule, covering the core scenarios of the two-model (original + effective) architecture. Also adds missing lombok, findbugs-annotations, and checker-qual dependencies to pom.xml — these were required at compile time because ArtifactV2.java uses Lombok annotations and lombok.config enables findbugs/checkerframework annotation generation.

Tests added

• Happy paths: version property used by direct dependency, managed dependency, plugin, managed plugin, extension, report plugin, and plugin dependency
• Violation paths: unused version property detected, multiple unused properties reported, mixed used/unused properties
• Filtering: non-.version properties are correctly ignored
• execute(): throws EnforcerRuleException on violations, passes cleanly when all properties are used

Review & Testing Checklist for Human

• Verify the added pom.xml dependencies are correct — lombok 1.18.36, findbugs-annotations 3.0.1, checker-qual 3.48.4 were added as provided scope. Confirm these versions are appropriate and that they were intentionally absent before (vs. inherited from a parent POM or supplied by an IDE).
• Check test coverage gaps — no tests for profile-scoped dependencies or for the parent-property-override edge case mentioned in the unusedUseViolation TODO. Decide if these are needed now.
• Run mvn test locally to confirm all 34 tests pass (note: requires Maven 3.9+ for JUnit 5 surefire support; Maven 3.6.3 bundled with Ubuntu won't work).

Notes

• Devin Session
• Requested by: @lithium147